I am not an expert, this is just my opinion. I agree, I think it's a lack of understanding of the interstellar medium. Currently, we have observed portions of deep state can vary in density between 0.0001 ions per cubic centimeter to a million ions per cubic centimeter, but this would be very hard to measure. If the average is on the higher end of the range, or higher than expected, we could vastly miscalculate the expected weight of normal matter given the sheer size of "empty" space. Furthermore abundance of heavy elements in the solar system point to a supernova event in our vicinity in the past which may have locally emptied/blasted away the particles around us, what if real space is less of a vacuum than we think and there is the missing mass.